The use of quadrature amplitude modulation ("QAM") and vestigial sideband ("VSB") modulation for transmitting data, such as high definition television ("HDTV") signals, is growing in popularity.
In the United States, HDTV systems, which include, e.g., Advanced television ("ATV") systems, appear to be headed in two different directions with regard to the type of modulation used to transmit television signals.
At present, cable companies in the United States are preparing to transmit HDTV digital signals over cable using a QAM technique of data transmission.
On the other hand, the terrestrial television industry in the U.S. is planning to broadcast digital HDTV, e.g., ATV signals, using a Vestigial SideBand ("VSB") technique of data transmission proposed by Zenith corporation/Grand Alliance.
Accordingly, to be capable of receiving and displaying both cable and terrestrial HDTV television transmissions, a television receiver will have to be capable of receiving and demodulating both QAM and 8 VSB HDTV signals.
Because the method used to demodulate QAM signals is different from the method used to demodulate VSB signals, a demodulator receiving a HDTV signal should be controlled so that it applies the correct demodulation method to a received signal. While the manual selection of the appropriate demodulation mode offers one solution to the problem, methods and apparatus for automaticly determining which of the two possible demodulation techniques is appropriate are desirable.
Furthermore, it is desirable that the automatic methods of determining the appropriate demodulation technique be able to distinguish between the two HDTV modulation formats without requiring demodulation of the received signal. In addition, the selection of the appropriate demodulation technique should generally be independent of transmission channel conditions, e.g., signal channel noise. HDTV signal channel noise may include, e.g., NTSC television signals broadcast by adjacent television transmitters in the same broadcast channel in which a HDTV signal is being broadcast.